The present invention relates to a gear pump or motor with high volumetric pump efficiency and, more particularly, to a gear pump or motor having an excellent oil seal between the gears and the housing.
Conventionally, a gear pump or motor comprises a pair of intermeshing rotatable gears mounted within a casing or housing. When one of the gears coupled to a main shaft is driven, oil will be fed from one port adjacent the intermeshing gears and exhausted out another port adjacent the intermeshing gears. This arrangement functions as a gear pump. When one port is connected to a high pressure oil supply and the other port receives oil at a lower pressure, the oil under pressure will be transmitted from a high pressure inlet to a low pressure outlet, thus driving the pair of gears. This mode functions as a motor in which the main shaft is rotated as an output shaft.
FIGS. 4 and 5 show a known construction of such a gear pump or motor in the mode of a gear pump, by way of example. A main shaft 1 and a follower shaft 2 are coupled to a pair of intermeshing spur gears 3 and 4, respectively. The gears 3 and 4 are contained within a casing 5 and positioned such that the tips of the teeth of the gears 3 and 4 slide on the inner surface of the casing 5. The main shaft 1 and the follower shaft 2 are rotatably mounted in bearings 8 and 9, respectively, disposed on a front cover 6 and a rear cover 7. Side plates 10 and 11 are interposed between the gears 3 and 4, and the front cover 6 and the rear cover 7 to stop oil leakage through the sides of the gears 3 and 4. In such an arrangement, when the main shaft 1 is rotated counterclockwise by a motor (not shown), oil will be fed from a low pressure port 12 adjacent a meshing area of the gears 3 and 4 and exhausted into a high pressure port 13 at the opposing meshing area of the gears 3 and 4 to thereby serve as a pump.
Conventionally, except where the ports are located, the gear tips and the sides of the gears 3 and 4 slide on the inner surfaces of the casing 5 and the side plates 10 and 11, respectively, so as to provide a small clearance therebetween which contains an oil layer. Thus, oil will be prevented from leaking around the gear teeth tips and the sides of the gears. In other words, the amount of oil leaking through the space formed between the meshing teeth of the gears 3 and 4, and between the side plates 10 and 11 and the gear sides will be minimized by providing an appropriate clearance between the gear teeth tips and the casing. To increase the efficiency of the gear pump or motor, the oil seal must be as small as possible at the sliding surfaces and is provided by selecting the necessary and the minimum clearance. Where the gear pump is operated as a high pressure pump, as in its recent wide use, since the oil exhaustion pressure increases, in particular in the high pressure pump, the leakage of the inner oil increases. If the oil seal is not sufficient between the sliding elements, the volumetric pump efficiency can be remarkably reduced during high pressure and temperature operation.
To manufacture a gear pump or motor in a known manner, in particular one suitable for use as a high pressure pump, the machining accuracy used in making the casing, the side plates, the bearings, the gears and the other components should be raised and these elements should be properly and selectively combined. In other words, to raise the machining and assembling measurement accuracy, it is necessary to provide the appropriate clearance between the gears and the peripheral walls of the casing. In particular, the inner diameter of the casing body is originally a little small so that a "running-in-operation" prior to the actual use of the device is conducted after the pump has been assembled. During the "running-in-operation", the inner surface of the casing body is cut by the hard tooth tips of the gears, whereby the clearance between the casing body surface and the tooth tips of the gear is adjusted. Even if there is some measurement error in the gear pump elements, the tooth tips of the gears will cut the inner surface of the case body, so as to automatically produce the appropriate clearance. Further, though the center of the gears may be deflected somewhat due to the pressure difference between the high pressure side and the low pressure side of the pump, the inner cutting of the casing by the gear teeth tips can effectively account for the deflection.
In this manner, the oil leakage from the high pressure side to the low pressure side of the pump can be prevented by the sliding between the cut inner surface of the casing 5 adjacent the low pressure port 12 and the tooth tips of the gears 3 and 4.
However, providing a clearance for oil sealing by cutting the casing has the disadvantage that when the operative condition of the gear pump or motor changes, in particular, the rotation speed and the temperature, the oil leakage at the gear teeth tips of the gears can be remarkably increased to thereby reduce the capacity or efficiency of the gear pump or motor. When the material of the casing 5 is a cast iron, the cutting face may be rough to thereby lower the seal efficiency and the volumetric pump efficiency of the gear pump or motor.
The present inventors find the following reasons why the oil leakage can increase when there is a change in the operation conditions. Where the oil seal is provided by cutting the inner surface of the casing, the lifting height of the shafts 1 and 2 due to the oil layer in the clearance is relatively small and the gears 3 and 4 are stressed to the low pressure side from the high pressure side. Therefore, as shown in FIG. 6, a center Og1 of each of the gears 3 and 4 is shifted by an amount equal to ex1 in the horizontal direction and by an amount equal to ey1 in the vertical direction with respect to a center Oj of each of the bearings. If the pump is operated in this condition, the gear teeth tips of the gear 3 are rotated with a radius of RgO around the deflected center Og1 against the inner surface of the casing 5 whose inner radius is Rb around the center Oj, adjacent the low pressure port 12 at the meshing area of the gears 3 and 4. Therefore, the inner surface of the casing 5 is cut as .epsilon.. When the pump is driven in the same condition, the gear teeth tips of the gear will slide on the casing 5 over the inner section S cut out of the casing 5 with an appropriate clearance to provide an oil seal.
Further, referring to FIG. 9, after the tooth tip 3a of the gear 3 is assembled with the inner surface of the casing or case body 5 to provide a uniform and appropriate clearance So, the "running-in-operation" is performed. After the "running-in-operation" as shown in FIG. 10, a portion of the case body 5 is selectively worn by sliding, so that eventually the case body 5 is worn in an arcuate shape with the greatest wear being at the central portion. After the "running-in-operation", if this pump or motor is used for high pressure operation, the gear 3 may be bent a relatively large amount as compared with the size of the case body 5 in response to the high pressure of the oil from the center of the side. In such a case, the clearance .beta. between the tooth tips 3a of the gear 3 at the ends and the inner surface of the case body 5 becomes very small as shown in FIG. 11. In conclusion, the clearance between the tooth tips 3a of the gear 3 and the inner surface of the case body 5 will be inevitably altered after the inner working due to the "running-in-operation" and the bending at the barrel center of the gear 3 during the high pressure operation. No matter how high the measurement accuracy of the components of the gear pump or motor is when the components are selectively combined, it cannot be expected that a uniform and appropriate clearance is maintained at the tooth tips 3a of the gear 3 during the high pressure operation.
As stated above, when the operative conditions of the gear pump or motor alter, particularly the rotation speed of the gears and the operation temperature, even once the appropriate clearance of the gear tooth tips is selected by the "running-in-operation", the oil leakage at the tooth tips can be remarkably increased because the deflection positions of the gear can be changed frequently and the inner surface portions cut by the gear tooth tips can be shifted also to thereby increase the gear tooth tip clearance.
As shown in FIG. 12, the gear 3 is supported by the journal bearing 8 in which the axis center of the gear 3 is deflected to be Og1 during low speed rotation and Og2 during high speed rotation with respect to the axis center Oj of the bearing 8. Where the axis center of the gear 3 is deflected because of an operational condition, the tooth tip 3a of the gear 3 cut the inner surface of the case body 5 in an inner radius Rg1 when the axis center of the gear 3 is Og1 and another inner radius Rg2 when the axis center of the gear 3 is Og2. In either of the deflection cases, the tooth tips 3a produce an excess clearance to thereby increase the oil leakage. Within the case body 5 having the inner radius of Rb, the tooth tips 3a cut a section B of the inner surface of the case body 5 in the low speed rotation, the section B being close to the low pressure port 12, and the tooth tips 3a cut another section C of the inner surface of the body 5 in the high speed rotation, the section C being far from the low pressure port 12.
In addition to the above-described disadvantageous result, the face of the inner surface of the case body which has then cut becomes rough and damages the volumetric pump efficiency.